In general, a direct positive silver halide emulsion is prepared by mixing a water soluble silver salt and a water soluble halide in a gelatin aqueous solution to form a silver halide grain (a core grain) and precipitating silver halide for forming a shell after subjecting the core grain to a chemical sensitization, followed by carrying out a desalting and a chemical sensitization according to necessity. In such a preparation, a parallel twin crystal-containing type tabular core/shell type direct positive silver halide emulsion is preferred in order to obtain high sensitivity.
Where a parallel twin crystal-containing type tabular silver halide emulsion is used for a core/shell type direct positive silver halide emulsion in a color diffusion transfer photographic film unit, the light-sensitive layer of a light-sensitive material can be made thinner. Because of this, the sharpness of the light-sensitive layer itself and the sharpness of the image transferred by diffusion can be improved. Further, a greater amount of a sensitizing dye can be adsorbed thereon since the surface/volume ratio thereof is large compared with a regular crystal grain, and therefore the sensitivity in color sensitization is improved. Because of this, graininess is improved as well. Accordingly, a color diffusion transfer photographic film unit can have high sensitivity and high image quality.
Meanwhile, in the case of a parallel twin crystal-containing type tabular core/shell direct positive silver halide emulsion, where a core grain is chemically sensitized and subsequently a silver salt solution and a halide solution are added at a low pBr, there has been the problem that the shell is not uniformly formed between the grains and the grain size distribution is expanded.
A parallel twin crystal-containing type tabular core/shell type direct positive silver halide emulsion is described in U.S. Pat. Nos. 4,395,478 and 4,504,570, and JP-A-1-297649 (the term "JP-A" as used herein means an unexamined published Japanese patent application) and JP-A-1-158429. In these cases, shell formation after chemically sensitizing a core grain is insufficiently even. Shell formation by adding a fine grain for the purpose of obtaining an even shell is disclosed in JP-A-1-183417 (U.S. Pat. No. 4,879,208). In this case, the evenness of the shell is insufficient and prevention of contamination by a multiple twin crystal grain was not investigated. In order to solve the problem accompanying the addition of a fine grain, a technique using a mixer and a protective colloid is disclosed in the above patent. However, the effects thereof are insufficient and in addition, the technique used is entirely different from the technique of the present invention.
Uneven shell formation in a core/shell type direct positive silver halide emulsion results in insufficient covering of the surface of the core grain. On the other hand, in the case of forming a shell by adding a fine grain for the purpose of achieving an even shell formation, while even shell formation is partially achieved, grains having therein no light-sensitive nucleus coexist, to which the fine grains themselves are grown. Thus, in either case, a low minimum density by reversal development is not achieved and particularly it is no longer practical for use in a diffusion transfer light-sensitive unit. Accordingly, the compatibility of the two is a problem that needs to be solved.